"Power factor correction" (PFC) techniques are often used in power converters to reduce the harmonic content of an input current by reforming the input current into approximately a sinewave. See U.S. Pat. No. 5,600,546 "INPUT HARMONIC CURRENT CORRECTED AC-TO-DC CONVERTER WITH MULTIPLE COUPLED PRIMARY WINDINGS", issued Feb. 4, 1997, Francis M. S. Ho, et al.; U.S. Pat. No. 5,446,366 "BOOST CONVERTER POWER SUPPLY WITH REDUCED LOSSES, CONTROL CIRCUIT AND METHOD THEREFOR", issued Aug. 29, 1995, John A. Bassett, et al.; and U.S. patent application "LOW COST AC-TO-DC CONVERTER HAVING INPUT CURRENT WITH REDUCED HARMONICS", Ser. No. 08/721,497, filed Sept. 27, 1996, Franki Ngai Kit Poon, et al.; the subject matter of each of these patent documents is incorporated herein by reference.
FIG. 1 is a simplified circuit diagram of an AC-to-DC power converter 2 designed to reduce the magnitude of input current harmonics with respect to the fundamental input current. AC-to-DC power converter 2 includes input terminals 4 for connection to an AC mains source of power; a rectifier bridge 6; a transformer 8 with a first primary winding 10, a second primary winding 12, and a secondary winding 14; a separate inductor 16; a storage capacitor 18; power diodes 20 and 22; a power switch 24; an output rectifier circuit 26; a control circuit 28; and output terminals 30 for outputting a DC current. In addition, an electromagnetic interference (EMI) filter 32 with a filter inductor 34 and a filter capacitor 36 is illustrated as coupled to power converter 2.
AC-to-DC converter 2 is inefficient due a large amount of input charging current flowing through separate inductor 16. FIG. 2 is a graph depicting the amount of current I.sub.16 flowing through separate inductor 16 relative to a rectified input voltage V.sub.R at node 7 where the horizontal axis represents time. The large amount of input charging current requires that inductor 16 have low resistance for efficiency reasons. The amount of input charging current flowing through inductor 16 also causes a problem with inductor 16 saturating because of the finite amount of magnetic flux that an inductor can carry in its core.